Directional microphone



March 4, 1952 B a BAUER 2,587,684

DIRECTIONAL MICROPHONE Filed OCT.. 15, 1948 2 SHEETS--SHEET 1 B. B.BAUER DIRECTIONAL MICROPHONE March 4, 1952 2 SHEETS-SHEET 2 Filed Oct.l5, 1948 j; /qooo INVENTOR fLz/amzfz E .Bauen persecond Such crystalsare Patented Mar. 4, 1,952

DIRECTIONAL MICROPHONE Benjamin B. Bauer, Oak Park, Ill., assigner toShure Brothers, -Incorp corporation-of Illinois orated, Chicago, Ill., a

Applicationv Cctoher 13, 1948, SerialNo. 54,221

23 Claims. 1

This invention 'relates `to directional sound translating devices, morefparticularly to directional microphones `having displacement responsive-sensitive elements, and `it is an object ofthe invention vto provideimproved Amicrophones of this character.

Directional microphones, that is, microphones which have lgreater-response for sound lreceived from one direction than 'from another andwhich utilize displacementresponsive elements such Vas piezoelectriccrystals are well known. One 'form of such microphones wherein va singletransducer or piezoelectric crystal is used and produces its outputthrough sound Veiects from two points in space is described and claimedin the Patent No. 2,237,298 to Benjamin Baumzwieger, now by judicialchange 'of name Benjamin B. Bauer, assigned to the same assignee as thepresent invention. -Microphones of the character described in the patentutilizing a single sound translating element, such as a diaphragm vwhichis acted upon by sound from two points in space, are essentiallyVpressure difference type microphones and when the diaphragm thereofdrives a displacement responsive element, "such as apiezoelectric'crystal, an output or response is obtained which increaseswith increasing frequency. 'This occurs because the pressure differencebetween the two points in 'space increases with frequency and `thedominant factor in controlling 'movements of the crystal is itsstiffness.

'Crystalshave natural frequencies of vibration or free resonances atwhich the amplitude of vibration is large. In `prior constructions (suchas described in thepatent referred to) the crystal dimensions have beenchosen so that the natural frequency of vibration :of the crystal .anddiaphragm is in the upper range of thefrequencies to vbe translated.Thus, for the normal frequency spectrum of about 100 cyclesiper secondto 10,000 cycles per second, the natural frequency of vibration may bein the vicinity of 4,500 to 6,000cycles quite'stiff, thereby requiring arelativelylarge ldriving force per-unit output voltage with consequentrelatively low output or sensitivity.

The rise in response at resonance (natural frequency) in the priordevicesis compensated for by providing a relatively small amount ofdamping such vaslafcloth placed incloseassociation with the diaphragm.Flat `or uniform response over substantially the whole -usefulAfrequency range ydespite the rising response withfincreasing frequencyis produced by vutilizing anelectrical compensating 'circuit or networkwhich introy duces-attenuation'increasing in proportion .to vthefrequency increase. Whilesuch a circuit maintains nat response, yit.produces a Yloss in lover-.al1 output .and keeps the output at the lowlevel determined by the circuit despite the higher out- :put available.At the higher frequencies the c'ircuit loses its eirectbut the :outputvremains substantially the same due todiffraction effects.

The dampingorresistance used inconnection with crystals and diaphragmsof the prior microphones .has .been small and .only prevented the risein .output 4at resonance, Vhigher .damping `to lObtain iat lresponsevover a greater frequency range being avoided becauseof theiresultantlow microphone output obtained and the diiculty of makingphysical-structureshaving the necessary acoustical properties.

Accordingly, V=it .is a further object Aof 'the invention to provide animproved microphone .of the character indicated having improved responseyor output level.

It is a further object of .the invention to provide an improvedv:microphone of the Vcharacter indicated wherein vflat or uniformresponse at .a higher output level is obtained vand the compensatingnetwork is eliminated.

Microphones of Vthe character described have been VlAmade directionalthrough .the use 'in each `unit `of -a phase Vshifting ynetwork which4applied ka force 4to one side 'of the fmicrophone diaphragm. While sucha phase shift network has resistance ordamping associated therewith, itdoes` not have fany substantial effect in Acontrolling the ampli-.element of the phase shift network.

VIt is a `further object vo1" the invention to provide anlimprovedmicrophone of the character described having high output level whereinthe 'dominant elementin controlling the diaphragm movements is the.damping component of the Vphase shift network.

Accordingto one form of the invention,a directional microphone for arange of frequencies is providedcomprising, a diaphragm adapted to be.substantially exposed-on one side to sound waves in a medium,aphaseshifting networkiincluding resistance and .a cavity or compliancebetween the other side of the diaphragmfand sound waves in the medium,piezoelectric means external to the cavity for producing an output,lever means coupling the diaphragm to the piezoelectric means, thenatural frequency of the system including the piezoelectric means, andthe diaphragm being in the upper range of the frequencies to betranslated, the lever means producing a natural frequency of the movingsystem including the piezoelectric means, the diaphragm and the leverapproximately equal to the geometric mean frequency of the frequencyrange to be translated, and the resistance or damping of the phase shiftnetwork being approximately equal to the reactance of the moving systemat tne lower range of frequencies to be translated.

For a better understanding of the invention, reference should be had tothe accompanying drawings in which:

Figure 1 is a front elevational view of the outer casing of a microphoneembodying the invention;

Fig. 2 is a sectional view taken substantially along lines 2-2 of Fig.1;

Fig. 3 is a perspective view of the microphone unit of Fig. 2 on alarger scale embodying the inventon;

Y Fig. 4 is a rear elevational view partly broken away of the unit shownin Fig. 3;

Fig. 5 is a sectional View on a larger scale taken substantially in thedirection of arrows 5-5 of Fig. 3;

Fig. 6 is a front elevational view of a modified microphone unitembodying the invention;

Fig. 'l is a sectional elevational view of a modified microphone unitembodying the invention;

Fig. 8 is a series of response curves for microphone units useful inexplaining the invention;

Fig. 9 is a sectional elevational view of a modifled microphone unitembodying the invention, and

Fig. 10 is a rear elevational view partially broken away takensubstantially in the direction ofY arrows Iii- I0 of Fig. 9.

Referring to the drawings, the invention is shown embodied in amicrophone unit I2 held in a grillwork casing I3 which comprises frontand rear grillV sections or halves I4 and I5', respectively, heldtogether by screws for example, not shown, and a swivel I6 pivoted tothe rear grill section. A stand (not shown) may be attached to theswivel and connecting leads may pass therethrough to the active part ofthe microphone unit. The casing and the grillwork may have any desiredconfiguration, and attached to the inside thereof are relatively porouscloth pieces I1 and I8 to provide wind shielding.

Microphone unit I2 is held to casing I3 by means of a strap 2I and ascrew 26, the strap being attached at its nrespective ends to transversemembers 22 and 23 which have pieces of sponge rubber 24 cemented orotherwise attached at their ends and in turn attached to brackets ofbase 25.

The microphone unit comprises a base 25 including a rearwardly formed(such as by punching) portionv 21, a generally conical curved diaphragm28, a piezoelectric crystal 3I cemented to the base, and a lever 32connecting the crystal and the apex of the diaphragm, leads 33 and 34extending from the crystal for bringing out the electrical output toterminals 35 and 36 (Fig. 4) respectively.

Diaphragm 28 is sealed around its edges to base member 25 therebyforming, together with depressed portion 21, a cavity 31, the diaphragmbeing made of thin material such as aluminum, stiff paper or the like.The edge of the diaphragm is made quite resilient or flexible so that itvibrates easily, but the diaphragm in general, including its apex, isquite rigid so that the diaphragm tends to vibrate as a whole and hassumcient rigidity to actuate (deflect) the crystal. The diaphragmrigidity, while the diaphragm is light, obtains mainly because of thecurved conical form.

Extending through depressed portion 21 is a series of holes 38 wherebysound waves at the rear of the base 25'may enter intovcavity 31 and thusact on the rear side of the diaphragm. Cloth 4I or other sound permeablematerial is placed over each of the holes for a purpose to be describedsubsequently in this specification. place of cloth which may be, forexample, silk with very ne interstices, other material having acousticresistance and inertance such as slits, diaphragms and the like may beused.

Crystal 3I is of the torsion type wherein an output is produced bytwisting thereof, one end of the crystal being relatively rigidlysupported across its full width on a. block of material 62 cemented tothe base and crystal. The other end of the crystal is relatively free sothat torsional movements may take place, but this end is lightlysupported on a second member- 43 which may be a resilient materialintroducing no substantial resistance to crystal movement. Torsion ofthe crystal is produced by lever 32 in transmitting movements of thediaphragm apex.

Lever 32 may be of any suitable shape and of any light material such asmetal (magnesium, for example) or plastic, and is shown in the form of atruss to lend the necessary rigidity. One end of the lever is coupled tothe diaphragm apex by any suitable means such as cementing and the otheris attached to both sides of the crystal such as by having the crystalreceived in notches, as seen best in Fig. 5. Hence as diaphragm 2Bvibrates back and forth the right end of the crystal is twisted back andforth, the twisting taking place about the center line of the crystal.

Crystal 3| may have a natural frequency when vibrating by itself abovethe highest frequency to be translated, but when coupled to a diaphragm,the natural frequency of the moving system including the crystal,diaphragm, and coupling structure may be in the upper range of soundfrequencies to be translated. For example, in a well known microphone,such as described in the patent referred to, the natural frequency ofthe crystal alone has been in the vicinity of 15,000 to 18,000 cyclesper second and the natural frequency of the crystal, diaphragm, etc. hasbeen in the Vicinity of 4,500 to 6,000 cycles per second. When such amicrophone operates on the principle of pressure difference, it has arelatively low output since the forces available for actuation aresmall.

In Fig. 8 there is drawn a curve 43 showing the response or output of apressure difference type crystal microphone such as where the naturalfrequency is inthe vicinity of 4,500 cycles per second, the frequencybeing shown on a logarithmic scale and the response being given indecibels (db.) with reference to an arbitrary output level. It will beseen that the response rises uniformly as the frequency increases untilin the vicinity of f2 where diffraction effects cause the pressuredifference to become constant the response levels olf, the responserises sharply in the vicinity of f0=4,500 cycles per second due to thenatural frequency, and the response falls olf rapidly above resonance.The frequency fz depends on the :ass-7,1684

transverse dimension v(outside diameter) of the unitfandin thevicinityof fav/here the transverse dimension `'is equalto vone-quarterwavelengthof sound the pressure tends to become constant. The sharp riseat resonance vmay be .prevented by a relatively smallamcunt .of dampingor resistance and results in the response vcurve following the dottedportion 44. The curve 43, .4.4 is onesuch as might be obtained with themicrophone shown in Fig. 6 of the rPatent No.v2,237,298 where the stiffcrystal is coupled directly to the diaphragm. With such-a'microphone ailatresponse may be obtaineolby using anV electrical-'attenuatingcircuit or network which begins functioning at some low frequency suchas 100 -cycles per second. Usingsuch a circuit, a-response folu lowingcurve 45 may 'be obtained which, whe

See Fig. 8 where fo is the natural frequency of the .moving systemwithout the lever, and f1 is the natural frequency of the moving systemwith the lever. L1 and L2 represent the lengths of the lever arms, asmay be seen from Fig. 5. It is a relatively simple matter to choose alever ratio such that the natural frequency of the moving system isreduced from 4,500 cycles per second to 1,000 cycles per .second or fromand to other values. The moving system includes in each unit thecrystal, the diaphragm and the lever or other coupling structure.

Changing the resonant frequency by changing the stiffness reflected onto the diaphragm results in a response curve for an undamped crystal,diaphragm and coupling structure which follows curve 4l of Fig. 8.Utilizing a lever as shown produces a greater deilection of the crystalfor the same pressure exerted on the diaphragm and thus the output orresponse is increased below resonance over the case Where no lever isused. The response also increases with increasing fren quency. Atresonance there is a sharp increase in response, and above resonance theresponse falls at one rate since the pressure difference available isstill increasing. This continues up vto f2 where diffraction effectscause the pressure `difference to become constant and the response fallsof at a more rapid rate.

By providing the resistance material 4I having acoustic resistance equalto the reactance of the moving system at a low frequency, for example100 cycles per second, the response curve following curve 48 may beobtained which is substantially fiat over the range from 100 cycles to10,003 cycles per second, i. e; substantially the 'useful frequencyrange. The response or output'shown by curve 48 is considerably abovethat of 46 and is accomplished through damping alone and without anattenuating network. It has been found that an average increase inresponse cf db. or more over the whole frequency range may be obtainedin this fashion:

It has been found that the preferred results are "obtained when thenatural or resonant frequency of Athe moving system is placedapproximately .at

the geometric mean .frequency of `the k*fret'ruency' range to betranslated, the geometric mean :frequencybeing the square-rootofthe.productief the lowest and.highestlfrequenciesto be translated. Thus ifthe lowestand highest frequencies to be translated are respectively `and.10,000 cycles per second, the geometric .mean frequency .is equal tov\/l00 v10,000or 1,000 cycles per second. It has likewise been foundthat good results fare obtained to a `considerable range oneachside ofthe geometric mean frequency such as down to one-half vthereof and up totwice thereof. It is also to `vbe understood that the frequencies :of100 and 10,000 chosen for the lower and yupper limits may bevaried'o-ver considerable `range as desired.

In the-Patent No. 2,237,298, it-has been shown that a microphonehavinga-diaphragmcoupled to a crystal wherein a phase shifting vnetworktransmits a force to the rear side of the diaphragm, acardioiddirectional response pattern is obtained when the volume of the cavityand the resistance and inertance comprising the network are-chosen sothat where The phase shifting network comprises the cavity 3l and theresistance and inertance of cloth 4|, and couples the rear side of thediaphragm ato ithesound pressure waves existing at the Yperforations 38.The delay or phase shift introduced by the network combined with theforce at the `front .surface of the diaphragm produces the directionalpattern. When the phase shift lproduced bythe network is equal tothat'experienced `bysound travelling .the effective distance from thefront of the diaphragm Yto -the network inlet for .normalfrontincidence, cardioid response is obtained.

In the instant application, referring to Fig. 5, d is the effectivedistance in centimeters from the front of diaphragm 23 to the openings3B, .and is closely approximated by the diameter of the-diaphragm plusthe thickness of case 25; Cv is as dened; C is the compliance of cavity31; R, and Lare the resistance and inertance of ma- -terial 4| in unitsas specified. While material 4i fhasinertance and resistance, it isprincipally resstancefto produce kthe proper Yphase shift.

kWhen the constants are chosen in accordance with .the'formulas given, acardioid directional response pattern is obtained in the subjectinvention similar to that of the patent.

In the structure of the ypatent the resistance of'thenetwork enteredmainly into shifting the lphase of .the force available at the rearnetwork, the diaphragm being damped at its natural frequency byresistance material placed at the front thereof. Furthermore, thecrystal was mounted inside of the cavity making it advisable to have thecavity large. Accordingly, the product RC was chosen equal to but with Rsmall and C large relatively. Then because of the rising response withincreasing frequency, the attenuating network was used to obtain a fiatresponse over the frequency range.

It has been found that when the natural frequency of the moving systemis approximately at the geometric mean frequency of the frequency rangeto be translated, the resistance R of the phase shift network may be sochosen relative to the volume of the cavity (which determines thecompliance C) that the phase shift for directional operation and fordamping the diaphragm at its natural frequency are obtained from thesame element. That is, the resistance R of material 4l may be chosen toaccomplish both damping and, in combination with the cavity andinertance of material Bl, phase shifting.

This in accomplished by choosing the product RC equal to choosing R inacoustic ohms equal to the reactance of the moving system at the lowfrequency to be translated and then choosing C to complete therelationship. The reactance of the moving system depends on its mass andstiffness and is relatively large at low frequencies. Hence theresistance component R is relatively large and in effect it becomes thedominant element in controlling the diaphragm movements. rIhat is, themoving system may be said to be resistance controlled. Consequently fromthe frequency at Which R equals the reactance of the moving system theresponse of the microphone is nat over the frequency range selected.

The quotient is readily found for any construction. Cv, the velocity ofsound in centimeters per second, is known, and d in units consistentwith Cv, for example centimeters, can 4be measured from the physicaldimensions of the structure.

To obtain resistances and reactances of the proper values, varioussamples of these elements are tested until elements of the proper Valuesare found according to generally known formulas and procedures.

Acoustical impedance in ohms may be dened as the complex quotient of thealternating pressure applied to the system and the resulting volumecurrent. See for example Elements of Acoustical Engineering, by Olson,page '73, second edition, published 1947 by D. Van Nostrand Company,Inc. rvlhe real part of this quotient is the resistance, and theimaginary part is the reactance. The definition of acoustical impedanceis sometimes stated as a quotient of sound pressure and particlevelocity. See for example an article entitled An acoustic transmissionline for impedance measurement published in The Journal of theAcoustical Society of America, volume 11, July 1939, page 142.

In the case of openings such as slits or the interstices of cloth, theresistance R is a function of the dimensions of the openings orinterstices. In slits which may have precise dimensions, the resistancemay b fairly well calculated by the expression Bauer, entitledConversion of Wave Motion into Electrical Energy and assigned to thesame assignee as the present application. In this expression:

R is the acoustic resistance in ohms,

u is the viscosity coefficient of the medium,

Z is the passage length in centimeters in the direction of flow,

t is the passage thickness in centimeters, and

L is the peripheral length yof the passage.

See also The Journal of the Acoustical Society, July 1931, volume 3,page 49.

When the resistance is cloth, the same formula applies, but there is apractical difliculty in determining the dimensions of the clothinterstices. For this reason the resistance may be measured by forcing astream of air, for example, through a sample of the material andmeasuring the pressure drop thereacross and the volume currenttherethrough. The resistance R in acoustic ohms may be given by theexpression Where p is the pressure drop in dynes per square centimeter,and o is the flow in cubic centimeters per second.

belge i Where A is the area of sample in square centimeters, p 1s thepressure drop in dynes per square centimeters V 1s the volume velocity mcubic centimeters per second.

In the article referred t0 as published in volume 11 of The Journal ofthe Acoustical Society of America, the impedance of an element may bedetermined by utilizing the element to terminate an already calibratedtube and supplying the tube with sound pressure of a particularfrequency. The impedance Z may be determined by the expression Pmax2TI'L r3.1) HQWTH The real part of the formula is the resistance Where Vand the imaginary part is .the reactance. The

method described may also-bensed.forfdetermining the impedance of cloth,butsince itis known thatcloth is primarily resistance, 4the methoddescribed in the O. S. R. D. publicationmay be used also.

Acoustical reactance of the mechanical components at anyvfrequency ofthe. crystal 3l, the lever 32 or other coupling structure, andthediaphragm-28 may be obtained by constructing such a combination andtesting it according to the procedure outlined in volume 11 of TheJournal of the Acoustical Society. of America.

Acoustical reactance is afunction ofthestiffnessA or rigidity (or the.inverse thereof, compliance) of the diaphragm, crystal and coupling, themass of these elements and the frequency of the sound. If the combinedelements are very stiff or rigid, then little sound will be transmittedthereby and theV impedance is high. Likewise, if the mass of theseelements vis great, little sound willbe transmitted thereby andtheimpedance is high. Conversely, if the combined elements are veryiiexible or compliant and of a very small mass, substantially all of thesound incident will be transmitted and the reactanceis low.

Experience has shown in general what dimensions of crystal to use inorder to obtain sufficient electrical output and the physical dimensionsand characteristics of a diaphragm to drive the crystal. It is notbelieved necessary to set out these criteria here. A particularselection-of crystal, diaphragm and coupling may be made and theimpedance thereof determined according to the procedure set out inthearticle in The C. and RC for other directional patterns is disclosed. Ithas equal application here and. thus is not given in detail.

Journal of the Acoustical Society of America,

volume 11. While the procedure there outlined will give both resistanceand reactance, it is known that the resistance of vsuch amechanicalsystem is very largely negligible compared to its reactance. Thereactance. having been determined at the lower frequency toA betranslated, a cloth having the requisite resistance may be selectedhaving this same value..

The resonant or natural frequency-of the combined crystal, coupling anddiaphragm may `be obtained by exposing the structure to sound of varyingfrequency and noting the frequency at which maximum vibration amplitudeoccurs.

In Fig. 8, curve 48 represents the case described, it having beenassumed that the resistance is equal to the reactance at. about 10Gcycles per second. Havingv chosen a resistance of this value, theresonant rise at 1,000 cycles per second is eliminated, the response isfiatover substantially the whole Ifrequency range, and the directionalpattern is that of a cardioid up to where the transverse dimensions ofthe casing are equal to one-quarter wavelength of the frequency beingtranslated. Thereafter diffraction effects maintain the directivity.Comparing curves 48 and d5, the output obtained by shifting the naturalfrequency of the system andv using the resistance of the phase shiftcircuit for damping is much higher and fiat response is obtained withoutthe use of an attenuating network.

While in the case described. a cardioid directional pattern was assumed,it is understood that any other directional pattern from pressureresponse to bi-directionalu or cosine law response may be obtained bylchoosing the product RC in known fashion relativeA to and thereafterchoosing R. to obtain the neces-V As seen best in Fig. 2, a second layerofcloth. 5|, which may be silk having relatively fine interstices, maybe placed across the front of the unit to improve the shielding out ofwindsound, etc. Thev support blocks 2li of sponge rubber or. the likeprevent substantially the transmissionF of mechanical vibrations totheunit.

Obtaining a moving systemhaving a natural frequency at the relativelylow frequency of. the geometric mean. of. the frequency range to.. betranslated may be .obtained by using avery thin or flexible crystal andapplying stresses to it di.- rectly without. the useof a lever. In Fig.6. there is shown a microphonev unit 52 having a crystal 53 of thischaracter. In .other respects,.that is, the diaphragm 54, the base55,.the cavity'and the cloth material (not shown), unit52 may be'substantially identical to unit i2. Crystal. 53 is thin and is placedunder bending stress rather than torsion, as may also be seen from .theconstruction of Fig. 7 showing a modification havinga Isimilar crystalbut differing in other respects; The crystal, being thin, has aV lownatural frequency of vibrationin combination with the diaphragm, butisquite fragile compared to the stiffer crystals already. described..

The resistance component for damping the diaphragm asv well as forphaseshifting may be obtainedby sound permeable means otherA than cloth.In Fig. 7 there is shown a structure wherein the resistance components Ris. obtained-,by the resistance to flow of air, the microphone unithavingr a case. 55, a diaphragm 51,. and a crystal 58.. The cavity 6|and.` diaphragm 51 maybe the same as. described for theother embodimentsand crystal 58. may be the` same as described for the embodiment ofVFig; 6'. The back of casing 5B' includes a series of openings orperforations. 60 between which` there are elements. of solid material62.' Snaced rearwardly from.` elements 62-is a thin membrane (-534 heldso spaced by, any suitable means suchv as by cement:- ing to a-circularridge, as shown.

Membrane 63 is a thin iiexible member through whichsound is transmittedwithout significant decrease in pressure over the frequency rangedesired. and may be madeY of foils of metals, plasticV materials,rubber, and the like. As the membrane. moves. back and forth under soundwaves,.air or other gasbetween the back surface ofmembers 52 andthefrontof the membrane is forced. to. iiow in and` out. Due to the viscosity ofsuchv air or. gas, its movement introduces resistance andinertance as isunder.- stood.. in this art. and provides the phase shift and damping..

In. this embodiment, the., cavity, .the resistance produced asdescribed, together with. the equivalent inertance introduced by the.membrane, forniv the phase shift network.. The...membrane is` an inletto. sound waves. since. with. proper choice: of.r characteristics. it.will, transmit. sound pressureY over. a. substantial range., of..frequencies without any significant reduction in the pres'- suremagnitude.

v'tion 86 provided with a series of perforations 61. Closing the frontend of depressed portion 66 is a diaphragm 68, and placed across theinside of perforations 61 is porous material 1I having acousticresistance and inertance. Diaphragm 68 and depressed portion 66 dei-lnea cavity 12 which, together with the resistance and inertance of porousmaterial 1l, provides a network for shifting the phase of pressurebefore it acts on the rear surface of the diaphragm in like manner tothe embodiments already described.

Coupled to the apex of diaphragm 68 is one end of a lever 13 having itsother end connected to a stationarily arranged crystal 14, also asalready described. Spaced away from the front surface of diaphragm 68 isa perforated metal 'supporting member 15 having porous material 18dening acoustic resistance and inertance attached to the inside surfacethereof, the space between the porous material and the diaphragmdefining a 'cavity 11.

Associated with the rear surface of depressed portion 66 is a shutter 18shaped, for example, as shown in Fig. 10, the perforations B1 beingarranged in a similar pattern. Shutter 18 is connected to a tube 8lwhich in turn is connected to a knob 82 by means of which shutter 18 maybe rotated. A stud 83 is firmly attached to the back of base 65 to forma support for member 8l and a. spring 84 is arranged as shown to holdshutter 18 firmly against the back of the casing.

Assuming in one instance that the front support member 15 and acousticmaterial 18 are not present, the microphone unit is similar to thatshown in Fig. except that the number of perforations forming an inletinto the microphone unit at the rear may be varied. This constructionmay be such that when all of the perforations are open, the microphonehas a cardioid directional pattern, and when shutter 18 completelyclosesall of the perforations the microphone is pressure responsive. Byvsuitably choosing the number and size of perforations 61, a

virtually continuous variation between these two patterns is obtainable.

When acoustic material 16 is being held adjacent the front surface ofdiaphragm B8, a second phase shifting network is added to the microphoneunit, this phase shifting network including the cavity 11 and theresistance and inertance of material 18'. The constants of this networkmay be so chosen that they are equal to the constants of the network atthe rear of the diaphragm whenever shutter 18 is completely open. Underthese conditions the same phase vshift is produced by the two networksand a cosine law or bi-directional response microphone results. As thelshutter 18 isclosed, the phase shift Yproduced by the rear networkincreases 'thereby producing a different directional pattern until whenshutter 18 is completely closed a pressure response microphone is had.This variation may be virtually continuous or in discreet steps,depending on the size and number ofthe perforations chosen.

In accordance with the teachings of this invention, the volume ofcavities 12 and 11 and the resistance of porous materials 1| and 18 maybe so chosen that the dominant factors in the control of the diaphragmmovements are the resistance components. By utilizing lever 13 thenatural frequency of the moving system may oe substantially at thegeometric mean frequency of the frequency rangeV to be translated.

Assuming, as previously, that the geometric mean frequency of thefrequency range is 1000 cycles per second and the lower frequency iscycles per second, the resistance of materials 1| and 16 may be chosento be substantially equal to the reactance oi" the moving system at thelower frequency. Approximately one-half of the resistance so obtainedmay be placed in each of the resistance materials. The-reactance of themoving system being relatively high at this low frequency, theresistance values will be high and thus the volumes of cavities 12 and11 may be made quite small. Consequently, the resistance of bothmaterials becomes the dominating element in controlling the movements ofthe diaphragm, the rise in response at the resonant frequency of 1,000cycles per second is eliminated, and a substantially fiat response overthe whole frequency range is obtained.

When the shutter 18 completely closes all of perforations B1 and themicrophone is pressure responsive, the natural frequency of the movingsystem is consideraly increased over that when the shutter is open. Thisfollows from the fact that when shutter 18 is closed the air in cavity12 acts as a greater stiffness (due to no escape through theperforations) which, in association with the diaphragm, the crystal andthe lever, results in the moving system having a frequency approximately4,000 to 6,000 cycles per second, whereas when shutter 18 is open thenatural frequency of the system may be 1,000 cycles per second. When themicrophone is operating as a pressure microphone, the resistance ofmaterial 16 acts to damp out the rise in response at resonance and tendsto produce a flat response over the frequency range, the response beingabout at the same level as that for the bi-directional case. It has beenfound that the same general level of output is obtained with otherdirectional patterns, that is, for positions of shutter 18 between fullyopen and fully closed.

Throughout thisv specication and claims the following definitions andconcepts obtain.

Where constants of the microphone are referred to, and relationshipsbetween the constants governing the operation are given, it isunderstood that the values thereof are in a consistent system of units,such for example as acoustic units.

A displacement responsive element is one producing an outputproportional to its displace ment or deflection from a normal orequilibrium position. Thus the voltage of crystal which is one form ofdisplacement responsive element is proportional to its deformation.

Where phase shift networks and the transmission o sound pressures arereferred to, it will be understood that the pressure does not changesignificantly in magnitude.

An acoustic phase shift network is one which shifts the phase ofacoustic pressure without regard to the character of the elementsproducing the phase shift. A network utilizing acoustic components isone where the components are gaseous elements such as cavities andphysical passages.

While particular embodiments of the inventionhavefbeen shown, it will'beunderstood, ci course, that the` invention is not limited thereto sincemanyVV modifications may be made, and it is, therefore, contemplated bythe appended claims to cover'any such modifications as fall withintheitrue spirit and scope of the invention.

Having thus described the invention, what is claimedv and desired to besecured by Letters Patent is:

l. A directional sound translating device for a range of frequenciescomprising, a translating unitadapted to-be exposed to sound pressurealong one portion thereof for producing an output proportional todisplacement, a sound pressure phase shifting network includingresistance and compliance forcoupling another portion of saidtranslating unit to sound pressure, said translating unit having anatural frequency of approximately the geometric means frequency ofthe'V range to be translated, and the resistance of said network beingapproximately equal to-the reactance of said translating unit atthelower end of the frequency range to be translated.

2. A directional sound translating device for a range-of frequenciescomprising, a translating elementadapted' to be substantially exposed tosound Waves: in a medium on one side thereof, a sound pressure phaseshifting network including resistance and compliance-between the otherside of said translating element and sound waves in said medium,transducing means producing an output proportional to displacementthereof, means coupling said transducing means to said translatingelement, said transducing means, said translating element and theassociated coupling structure having a natural frequency approximatelyequal to the geometric mean frequency of the frequency range tobetranslated, and the re sistance of said network being approximatelyequal tothe reactance of the translating element, thel transducing meanscoupled thereto, and the associated coupling'structure at'the lower endof the frequency range to be translated, thereby to effect substantiallyconstant output from said transducing means over said frequency irange.

3. A directional sound translating device for a rangev of frequenciescomprising, a diaphragm adapted to be substantially exposed to soundwaves in a medium on one side thereof, a sound pressure phase shiftingnetwork including resistance and compliance between the other side ofsaid diaphragm and sound waves in said medium, transducingmeansproducing an output proportional to displacement thereof, means couplingf said transducing means to said diaphragm, said transducing means, saiddiaphragm and the associated coupling structure having a naturalfrequency approximately equal' to the geometric meanl frequency of thefrequency range to be translated, the resistance of said network beingapproximately equal to the reactance of the diaphragln, the transducercoupled thereto, and the associated coupling structure at thelower endofthe frequency range to be translated.

Ll. A directional sound translating device for a range of frequenciescomprising, a diaphragm adapted to bev substantially exposed to soundwaves in a medium on one side thereof, a sound pressure phase shiftingnetwork having acoustic compliance and acoustic resistance between theotherfside ofsaid diaphragm and sound waves in said medium,transducingmeans producing an output.4 proportional; to displacement.thereof, means couplingv said.l transducing means toi said coupledthereto, and the associated coupling structure at theV lower end of thefrequency range to betranslated, thereby to effect substantiallyconstant output from said transducing means over said frequency range.

5. A. directional sound translating device for a range of frequenciescomprising, a diaphragm adapted to be substantially exposed to soundwaves in a medium on lone side thereof, a sound pressure phase'shiftingnetwork having acoustic compliance and acoustic resistance between theother side of said diaphragm and sound waves in said medium,piezoelectric means for producing an output, means for coupling saidpiezoelectric means to said diaphragm, said piezoelectric means', saiddiaphragm and the associated coupling structure having a naturalfrequency approximately equal to the geometric mean frequency of thefrequency range to be translated, and the resistance of said networkbeing approximately equal to the reactance of said piezoelectric means,said diaphragm and said associated coupling structure at the lower endof the frequency range to be translated.

6. A directional sound translating device for a range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyexposed to sound waves in a medium on one side thereof, a sound pressurephase shifting network including acousticV resistance and acousticcompliance between the other side of said diaphragm and sound waves insaid medium, transducing means for producing an output proportional tothe displacement thereof, means for connecting said transducing meanssubstantially directly to the apex of said diaphragm, said transducingmeans, said connecting means and said diaphragm havingA a naturalfrequency approximately equal to the geometric mean frequency of thefrequencyrange to be translated, and the resistance of said networkbeing approximately equal to the' reactance of the transducing means andthe diaphragm at the lower end of the frequency range to be translated.

'7. A directional sound translating device for a range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyeX- posed to sound waves in a medium on one side thereof, a. soundpressure phase shifting network including resistance and compliancebetween the other side of said diaphragmand soundv waves in saidmedium,transducing means for producing an output proportional to thedisplacement thereof,

said transducing means and diaphragm having a natural frequency withinthe higher range of frequencies to be translated, lever means couplingthe apexof said diaphragm to said transducing means, said lever meansproducing a natural frequency of rthemoving system including saidtransducing means, said diaphragm and said lever means approximatelyequal to the geometric mean frequency of the frequency range to betranslated, and the resistance of said network being approximately equalto the reactance of said moving syse tem atthe lower end. of thefrequency range to be translated.

andirection'allsound translating device; for a range/of. frequencies.comprisinga diaphragm inl cluding an apex adapted to be substantiallyexposed to sound waves in a medium on one side thereof, a sound pressurephase shifting network including acoustic resistance and acousticcompliance between the other side of said diaphragm and sound waves insaid medium, transducing means for producing an output proportional tothe displacement thereof, said transducing means and diaphragm having anatural frequency within the upper range of frequencies to betranslated, lever means coupling the apex of said diaphragm to 'saidtransducing means, said lever means producing a natural frequency of themoving system including said transducing means, said diaphragm and saidlever means approximately equal to the geometric mean frequency of thefrequency range to be translated, and the resistance of said networkbeingr approximately equal to the reactance of said moving system at thelower end of the frequency range to be trans- L lated.

9. A directional sound translating device for a range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyexposed to sound waves in a medium on one side thereof, a sound pressurephase shifting network including acoustic resistance and acousticcornpliance between the other side of said diaphragm and sound waves insaid medium, piezoelectric means for producing an output, saidpiezoelectric means and diaphragm having a natural frequency within theupper range of frequencies to be translated, lever means coupling theapex of said diaphragm to said piezoelectric means, said lever meansproducing a natural frequency of the moving system including saidpiezoelectric means, said diaphragm and said lever means approximatelyequal to the geometric mean frequency of the frequency range to betranslated, and the resistance of said network being approximately equalto the reactance of said moving system at the lower end of the frequencyrange to be translated.

10. A directional sound translating device for a -range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyexposed to sound waves in a medium on one side thereof, a sound pressurephase shifting network including acoustic resistance and acoustic'compliance between the other side of said diaphragm and sound waves insaid medium, piezoelectric Vmeans for producing an output, saidpiezoelectric means having a natural frequency within the higher rangeof frequencies to be translated, lever means coupling the apex of saiddiaphragm to said piezoelectric means, said lever means producing anatural frequency of the moving system including said piezoelectricmeans, said diaphragm and said lever means approximately between onehalf of and twice the geometric mean frequency of the frequency range tobe translated, and the resistance of said network being approximatelyequal to the reactance of said moving system at the lower end of thefrequency range to be translated.

.11. A directional sound translating device for a range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyexposed to sound waves in a medium on one side thereof, structure at theother side of said diaphragm forming a cavity, a sound inlet to saidcavity, acoustic resistance means associated with said inlet, saidcavity and said resistance means comprising a phase shift network forsound pressure.,transducing means for producing an output having anatural frequency within the upper frequency range to be translated,lever means coupling the apex of said diaphragm to said transducingrmeans, said lever means producing a natural frequency of the movingsystem including said transducing means, said diaphragm and said levermeans approximately equal to the geometric mean frequency of thefrequency range to be translated, and the resistance of said resistancemeans being approximately equal to the reactance of the moving system atthe lower end of the frequency range to be translated.

l2. A directional sound translating device for a range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyeX- posed to sound waves in a medium on one side thereof, structure atthe other side of said diaphragm forming a cavity, a sound inlet to saidcavity, acoustic resistance means associated with said inlet, saidcavity and said resistance means comprising a phase shift network forsound pressure, transducing means for producing an output having anatural frequency within the upper frequency range to be translated,lever means coupling the apex of said diaphragm to said transducingmeans, said lever means producing a natural frequency of the movingsystem including said transducing means, said diaphragm and said levermeans approximately between one half of and twice the geometric meanfrequency of the frequency range to bue translated, and the resistanceof said resistance means being approximately equal to the reactance ofsaid diaphragm, said transducing means, and said lever means at thelower end of the frequency range to be translated thereby to effectsubstantially constant output from saidtransducing means over saidfrequency range.

13. A directional sound translating device for a range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyexposed to sound waves in a medium on one side thereof, structure at theother side of said diaphragm forming a cavity, a sound inlet to saidcavity, acoustic resistance means associated with said inlet, saidcavity and said resistance means comprising a phase shift network forSound pressure, transducing means external to said cavity for producingan output having a natural frequency within the upper frequency range tobe translated, lever means coupling the apex of said diaphragm to saidtransducing means, said lever means producing a natural frequency of themoving system including said transducing means, said diaphragm and saidlever means approximately equal to the geometric mean frequency of thefrequency range to be translated, and the resistance of said resistancemeans being approximately equal to the reactance of said diaphragm, saidtransducing means, and said lever means at the lower end of thefrequency range to be translated thereby to effect substantiallyconstant output from said transducing means over said frequency range.

14. A directional sound translating device for a range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyexposed to sound waves in a medium on one side thereof, structure at theother side of said diaphragm forming a cavity, a sound inlet to saidcavity, acoustic resistance means associated with said inlet, saidcavity and said resistance means comprising a phase shift network forsound pressure, piezoelectric means external to said cavity forproducing an output having a natural fretranslated, lever means couplingthe vapex of said diaphragm to said piezoelectric means, said levermeans producing a natural frequency of the moving system including saidpiezoelectric means., 'said diaphragm and said lever means approximatelyvequal to the geometric mean frequency of the frequency range to betranslated, and the resistance of said resistance means beingapproximatelyv equal to the reactance vof said diaphragm, saidpiezoelectric means, and said lever means at the lower end of thefrequency range to be translated thereby to effect substantiallyconstant output from said piezoelectric means over said frequency range.

A directional sound translating device for a range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyexposed to soundwaves in a medium on one side thereof, structure at theother side of said diaphragm forming a cavity, a sound inlet to saidcavity, acoustic resistance means Aassociated with said inlet, saidcavity and said resistance means comprising a phase shift network forsound pressure, piezoelectric means for producing an output having anatural frequency within the upper frequency range to be translated,lever means coupling the apex of said diaphragm to said piezoelectricmeans, said lever means producing a natural frequency of the movingsystem including said piezoelectric means, said diaphragm and said levermeans approximately between one half of and twice the geometric meanfrequency ofthe frequency range to be translated, and the resistance vofsaid resistance means being approximately equal to the reactance of saiddiaphragm, saidv piezoelectric means, and said lever means. at thelower'end of the frequency range to be translated thereby to effectsubstantially constant outputfrom said piezoelectric means over saidfrequency range.

16. A directional sound translating device for a range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyexposed to sound waves in a medium on one side thereof, structure at theother side of said diaphragm forming a cavity, sound permeable meansforming an inlet to said cavity and defining acoustic resistance andinertance, the compliance of said cavity and the resistance andinertance of said permeable means forming a phase shifting network forsound pressure, piezoelectric means external to said cavity forproducing an output, said piezoelectric means having a natural frequencywithin the upper frequency range to be translated, lever means couplingthe apex of said diaphragm to said piezoelectric means, said lever meansproducing a natural frequency of the moving system including saidpiezoelectric means, said diaphragm and said lever means approximatelyequal to the geometric mean frequency of the frequency range to betranslated, and the re -sistance of said sound permeable means beingapproximately equal to the reactance of said diaphragm, saidpiezoelectric means, and said lever' means at the lower end of thefrequency range to be translated thereby to eiect substantially constantoutput from said piezoelectric means over said frequency range.

17. A directional sound translating device for a range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyexposed to sound waves in a medium on one side thereof, structure at theother side of said diaphragm forming a cavity, passage means into 18said cavity, porous material defining principally acoustic resistancecovering said passage means, piezoelectric means external to said cavityfor producing an output, said piezoelectric means having a naturalfrequency within the upper frequency range to be translated, lever meanscoupling the apex of said diaphragm to said piezoelectric means, said-lever means producing a natural frequency of the moving systemincluding said piezoelectric means, said diaphragm and said lever meansapproximately equal to the 'geometric Vmean frequency of the frequencyrange to `be translated, and the' resistance of said porous materialbeing approximately equal to the reactance of said diaphragm, saidpiezoelectric means, and said lever means at the lower end of thefrequency range to be translated thereby to effect substantiallyconstant output from said piez^electric noeans over said frequencyrange.

18. A directional sound translating device for a'range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyexposed to sound waves in a medium on one side thereof, structure at theother side of said diaphragm forming a cavity, sound permeable meansforming an inlet to said cavity and defining acoustic resistance andinertance, the compliance ofsaid cavity and the resistance and inertanceof said permeable means forming a phase shifting network for soundpressure, the resistance and inertance of said sound permeable means andthe compliance of said cavity being chosen so that the phase shiftproduced'thereby is substantially equal to the phase change produced insound pressurebetween said one side of said diaphragm and said inlet fornormal front sound incidence, piezoelectric means external to saidcavity for producing an output, levermeans coupling the apex ofv saiddiaphragm to said piezoelectric means, said lever means producing anatural frequency ofthe moving system including said piezoelectricmeans, said diaphragm and said lever means approximately equal to thegeometric mean frequency of the frequency range to be translated, andthe resistance of said sound permeable means further being approximatelyequal to the reactance of said moving system at the lower end of thefrequency range to be translated thereby to eifect substantiallyconstant output from said piezoelectric means over said frequency range.

19. A directional sound translating device for a range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyexposed to sound waves in a medium on one side thereof, structure at theother side of said diaphragm forming a cavity, sound permeable meansforming an inlet to said cavity and dening acoustic resistance andinertance, the compliance of said cavity and the resistance andinertance of said permeable means forming a phase shifting network forsound pressure, piezoelectric means external to said cavity forproducing an output, said piezoelectric means having a natural frequencywithin the upper range to be translated, lever means coupling the apexof said diaphragm to said piezoelectric means, said lever meansproducing a natural frequency of the moving system including saidpiezoelectric means, said diaphragm and said lever means approximatelyequal to the geometric mean frequency ofthe frequency range to betranslated, means for varying said sound permeable means thereby toeiect different output directional patterns, and the resistance of saidsound permeable means beadsense I9 ing approximatelyequal tothe-reactance of sai diaphragm, said piezoelectric means, and said levervmeans at the lower endof4 the frequency range to be translated therebyto effect substantially constant output from "said piezoelectric meansover said frequency range. f

20. A directional sound translating device for a range of frequenciescomprising, a diaphragm including an apex adapted to be substantiallyexposed to sound waves in a medium on one side thereof, structure at theother side of said diaphragm forming acavity, a thin diaphragm formingsound inlet to said cavity,` a perforated plate closely spaced to saidthin diaphragm, said thin diaphragm having equivalent inertance and airmovement between said thin diaphragm and perforated plate deningacoustic resistance, the compliance of said cavity and said resistanceand inertance forming a phase shifting network for sound pressure,piezoelectric means external to said cavity for producing an output,said piezo.- electric means having a natural frequency within the upperrange to be translated, lever means cou;

pling the apex of saiddiaphragm to said piezoelectric means, said levermeans producing a natural frequency of themoving systeml including saidpiezoelectric means, said diaphragm and said lever means approximatelyequal to the geometric mean frequency of the frequency range vto betranslated, and said resistance being approximately equal to thereactance of said diaphragm, said piezoelectric means, and said levermeans at the lower end of the frequency range to be translated therebyto effect substantially constant output from said piezoelectric meansover said frequencyrange. .q

21. A directional soundtranslating device -fora. range offrequenciescomprising, a diaphragm including an apex, first porousmaterial having acoustic resistance and inertance spaced from one sideof said diaphragm, second` porousmaterial having acousticresistance andinertance spaced from the other side of said diaphragm, saidfirst andsecond porous materials defining first and second cavities and with saidrespective .porous materials defining first and secondv sound pressurephase shifting networks having approxi;

means external to said cavities for producing an output, saidpiezoelectric means having av natural frequency within the upperfrequency rangev to be translated, lever-means coupling the apex of saiddiaphragm'to said piezoelectric means, said lever means producing anatural frequency of the moving system including said piezoelectricmeans, said diaphragm and said lever means approximately equal to'thegeometric mean frequency of the frequency range to be translated, thetotal resistance of both said porous materials being approximately equalto the reactance of said moving system at the lower end of the frequency'range to be translated, and means for varying said second network fromphase shift equality with 'said first network to closing oi the otherside of said diaphragm. l 22. The invention as defined in claim 21wherein thephase shift of said second network may be variedsubstantially continuously.

23. The invention as defined in claim 21 wherein the phase shift of saidsecond network may be variedin discrete steps.l

, BENJAMIN B. BAUER.

, REFERENCES CITED The'follo'wing references are of record in the fileof this patent:

UNITED STATES PATENTS Great Britain Nov. 1'1, 1947`

